Cold Cathode Fluorescent Lamps (CCFL) generally comprise a tube containing an inert gas or a mixture of inert gases and a small quantity of mercury. A pair of complementary electrodes are sealed at opposite ends of the tube in order to supply electrical current through the tube, and a small quantity of an electron emissive material is coated on the surface of the electrodes in order to promote the emission of electrons. When a sufficiently high voltage is applied across the lamps, by means of the electrodes, the electric field established causes some of the electrons within the inert gas and mercury vapour to become accelerated in the direction of the electrodes. Some of the electrons and ions thereby created reach the electrodes with sufficient kinetic energy to cause the electrodes to become heated to emit more electrons partially by the mechanism of field emission and partially by thermionic emission. As the process continues and more and more electrons become created within the lamp volume the electrodes become heated to a point where the electron emission process from the cathode is mainly thermionic and the amount of energy required to sustain the electric discharge created through the lamp becomes substantially reduced i.e. the gas/vapour has become ionised. The ultra violet light generated by the discharge in the ionised gas/vapour in turn excites the phosphorous coating on the tube to emit white/visible light
The electrodes generally used within cold cathode devices, for example, neon sign lamps, gas lasers and fluorescent lamps generally comprise a metallic cup-shaped or tube-shaped container and the emissive coating usually consists of a thin coating on the inner surface of the cup or tube.
During the lamp starting process the so-called “glow to arc” transition occurs, where the discharge initially goes from a condition of high localized fields in the vicinity of the electrodes until the electrodes become heated to thermionic emission and to a condition of relatively low energy localized fields in the vicinity of the electrodes when the lamp is in its operational arc discharge mode. During the condition of high localized fields in the vicinity of the cathode the entire electrode structure including the coating is continuously bombarded by relatively energetic electrons and ions until the thermionic emission process occurs. During this period of bombardment a quantity of the emissive coating becomes sputtered away and by this mechanism upon successive starting and “glow to arc” transitions the emissive coating becomes consumed until after a successive number of starts there is no longer sufficient emissive coating to supply electrons to the discharge so that the electrode becomes “deactivated” and the lamp is no longer operational.
CCFL's of a kind as for example shown in FIG. 1 are commonly used for providing back light in scanners, photocopiers and fax machines, and more importantly and recently in LCD monitors/televisions. An important sought-after characteristic of an LCD monitor/television is its lifetime, which depends largely on the lifetime of the CCFL used therein. Many factors can reduce the CCFL's lifetime. For example reduction in the amount of mercury in the tube, changes to the fluorescent powder, deterioration of the glass tube, increases in the amount of waste gases in the tube and the general “aging” of the electrodes.
One problem with the current CCFL is that sputtering occurs when the electrons bombard a small surface area at the end of the electrode (cathode) farthest into the tube (FIG. 6). The electrodes of a CCFL commonly used are mostly tube-shaped (FIG. 1 and FIG. 2). The internal diameter of the glass tube is approximately from 1 to 8 mm, so the diameter of the electrode is approximately from 0.7 to 7 mm.
Two parallel metal plates are also commonly used as an electrode (FIG. 3). A third possibility is a rod-shaped electrode (FIG. 4).
For both the tube-shaped and the parallel plate electrodes, multiple electron emission is possible (see FIG. 5). One result of sputtering is that it causes metal to be collected on the fluorescent powder or the inner wall of the glass tube.
Sputtering will reduce the brightness of the lamp because of the metal “coating” on the wall. The metal collected on the wall will also present a secondary conducting path for the electrons (see FIG. 11). The secondary conducting path may cause emission of waste gases from the glass and eventual breakage of the glass tube.